Growth of single crystal nanowires

ABSTRACT

The present invention provides nanowires which are substantially straight and substantially free of nanoparticles and methods for making the same The nanowires can be made by seeded approaches, wherein nanocrystals bound to a substrate are used to promote growth of the nanowire. Nanocrystals in solution may also be used to make the nanowires of the present invention. Supercritical fluid reaction conditions can be used in a continuous or semi-batch process.

STATEMENT OF GOVERNMENT RIGHTS

This invention was made with Government support from the NationalScience Foundation under Contract No. CTS-9984396. The government hascertain rights in this invention.

CROSS-REFERENCE TO RELATED PATENT APPLICATIONS

This application claims priority to U.S. patent application Ser. No.10/883,966, filed Jul. 6, 2004, bearing the same title and having thesame inventors, allowed now issued as U.S. Pat. No. 7,335,259 on Feb.26, 2008, which is incorporated herein by reference in its entirety, andwhich claims priority to provisional application No. 60/485,244, filedJul. 8, 2003.

BACKGROUND

Nanowires can be useful in electronic and optical devices includingintegrated circuit interconnects, field effect transistors,photodetectors, biochemical sensors, light-emitting diodes, andcomplementary logic devices. Synthetic methods are known which do notrely on conventional lithographic techniques which yield siliconnanowires micrometers in length with diameters below 10 nm. Thesynthetic methods, however, are not necessarily commercially attractive.Thus, a need exists for a method for producing nanowires, includingGroup IV nanowires including silicon and germanium nanowires, which areof commercial interest. They should feature, for example, improvedsynthetic and morphological control and commercial productioncapabilities for providing high quantities of materials. In particular,a need exists for a method of producing nanowires which high quality:for example, they should be substantially straight and substantiallyfree of corresponding nanoparticles, which can form along with thenanowires. In addition, material stability can be important, and surfacepassivation methods are also needed which are compatible with thenanowire synthesis.

SUMMARY

The present invention provides multiple embodiments which are summarizedin this section. However, the scope of the present inventions should notbe limited by the scope of this summary section.

For example, the present invention provides a process for growingcrystalline Group IV metal nanowires comprising: providing a substratecomprising catalyst sites attached to the substrate surface, andcontinuously reacting the catalyst sites with a supercritical fluidmixture comprising at least one Group IV metal precursor, wherebycrystalline Group IV metal nanowires grow from the catalyst sites. Thecatalyst sites can comprise nanoparticulates such as, for example,metallic nanocrystals. The attachment can be, for example, a covalentattachment. A cross sectional flow rate for the supercritical fluidmixture over the substrate and a reaction temperature can be controlledto produce nanowires which are substantially straight nanowiressubstantially free of nanoparticles.

In another embodiment, the present invention provides a process forpreparing semiconductor nanowires comprising: growing Group IV metalnanowires under supercritical fluid conditions with use of seed catalystparticles, wherein the seed catalyst particles are attached to asubstrate.

In another embodiment, the present invention provides a process forpreparing Group IV metal nanowires substantially free of nanoparticlesconsisting essentially of: growing Group IV metal nanowires undersupercritical fluid conditions with use of seed nanocrystals, whereinthe seed nanocrystals are attached to a substrate, and a supercriticalmixture is passed over the seed nanocrystals.

In another embodiment, the present invention provides a process forproduction of nanowires comprising: first passing a supercritical fluidmixture comprising a solvent and a group IV metal precursor over asubstrate in the presence of catalyst seed particles, whereby nanowiresare deposited onto the substrate; and second passing a washing solventover the substrate to remove the nanowires from the substrate.

In another embodiment, the invention comprises a process for growingcrystalline Group IV metal nanowires comprising: providing a dispersioncomprising at least one Group IV metal precursor and nanoparticulatemetallic catalyst, and continuously reacting the dispersion undersupercritical fluid conditions, whereby crystalline Group IV metalnanowires grow from the nanoparticulate metallic catalyst.

In another embodiment, the invention comprises a process for growingpassivated crystalline Group IV metal nanowires comprising: providing adispersion comprising at least one Group IV metal precursor, at leastone passivation agent, and nanoparticulate metallic catalyst, andcontinuously reacting the dispersion under supercritical fluidconditions, whereby crystalline Group IV metal nanowires grow from thenanoparticulate metallic catalyst.

In another embodiment, the invention comprises a process for growingcrystalline Group IV metal nanowires comprising: providing a dispersioncomprising at least one Group IV metal precursor and at least oneprecursor for nanoparticulate metallic catalyst, and continuouslyreacting the dispersion under supercritical fluid conditions to formGroup IV metal and nanoparticulate metal catalyst, whereby crystallineGroup IV metal nanowires grow from nanoparticulate metallic catalyst.

The invention, furthermore, provides a method of surface passivation ofnanowires comprising: preparing nanowires with use of a supercriticalfluid process in a reaction cell; passivating the nanowires withoutopening the reaction cell.

Articles and devices comprising nanowires, which are prepared by thesemethods, are also part of the invention.

The invention also provides compositions of matter including crystallinenanowires comprising group IV metal which are substantially straight andsubstantially free of nanoparticles.

Advantages provided by the nanowires of the present invention include:(1) the ability to prepare substantially straight nanowires; (2) theability to prepare nanowires substantially free of nanoparticles; (3)the continuous production of nanowires, in quantities that areeconomically attractive; (4) the ability to functionalize the nanowiresurfaces by chemical treatment without exposure to oxidizing atmosphericconditions; (5) the ability to control nanowire size; and (6) theability to control kinetic parameters for process optimization. Theability to produce straighter, more uniform, defect-free nanowiresrepresents a significant improvement over other presently availablenanowires because they provide superior electronic properties for deviceapplication and pack easily into well-ordered two- or three-dimensionalarrays. Examples of suitable devices in which the nanowires may be usedinclude, but are not limited to field effect transistors, lasers,photodetectors, biochemical sensors, light emitting diodes,complementary logic devices and non-volatile memory devices.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A-D provides a schematic of a nanowire synthesis from tetheredsterically stabilized nanocrystals.

FIG. 2 provides a schematic of the flow reactors which can be used tosynthesize nanowires.

FIG. 3A-C provides images and XPS analysis of a substrate after graftingnanocrystals to the surface.

FIG. 4A-F provides images of nanowires grown from nanocrystals on asubstrate in a batch reactor.

FIG. 5A-D provides images of nanowires synthesized in a flow reactor.

FIG. 6 provides HRTEM image of nanowires produced in the flow reactor.

FIG. 7 provides an illustration of the surface derivatization of Genanowires by thermally initiated hydrogermylation according to Example2. Heating the nanowires in the presence of 1-alkene forms an alkylterminated Ge nanowire surface.

FIG. 8 provides an HRTEM image of a clean Ge nanowire surface aftertreatment with 1-hexadecene according to example 2. EDS analysis in theTEM showed no increase in oxygen content during the course of 1 week.

FIG. 9 provides an FT-IR spectrum of the Ge nanowire sample afterderivatization according to example 2. The Ge nanowire pictured in FIG.8 comes from this same sample.

FIG. 10 provides an FT-IR spectra of Ge nanowires produced by continuousflow synthesis with simultaneous in situ surface passivation using1-hexene according to example 4. The peak at 2965 wavenumbers due to C—Hbonds indicates surface passivation with hexyl groups.

FIG. 11 provides an FT-IR spectrum of Ge nanowires produced bycontinuous flow synthesis with simultaneous in situ surface passivationusing 1-hexene according to example 4. The peak at 860 wavenumbers isdue to Ge—O bonds and indicates some surface oxidation.

FIG. 12 provides HRTEM images of Ge nanowires produced using continuousflow synthesis with simultaneous in situ surface passivation accordingto example 4.

FIG. 13 provides an electron energy loss spectrum (EELS) of the Genanowire produced using continuous flow synthesis with simultaneous insitu surface passivation according to example 4.

DETAILED DESCRIPTION OF THE INVENTION

Methods are provided for synthesizing nanowires by thermally degrading aGroup IV organometallic precursor in a supercritical fluid in thepresence of nanocrystals or nanoparticulates, or nanocrystal precursorsor nanoparticulate precursors. The processes can be continuous orsemi-batch processes. The nanocrystals or nanocrystal precursors can betethered to an appropriate substrate surface or dispersed freely insolution. The resulting nanowires may be characterized by narrowdiameters and size distributions. The use of supercritical fluids allowsfor the dispersion of high concentrations of nanocrystals and precursorsin the fluid while maintaining high diffusion coefficients. Thiscombination leads to fast and efficient nanowire production. Moreover,the ability to manipulate precursor concentration, nanocrystalconcentration and size, and the solvent strength of the supercriticalfluid via temperature and pressure provides flexibility in controllingthe nanowire composition and morphology.

The inventors have surprisingly and unexpectedly discovered that byutilizing a flow reactor in combination with tethered nanocrystals inthe production of the nanowires, substantially straight andsubstantially defect-free nanowires may be manufactured withoutsacrificing nanowire length and without the production of significantamounts of nanoparticles which are often produced in competitivereactions during nanowire production. The flow reactor provides kinetictunability that minimizes undesirable particle deposition and optimizesthe production of straight nanowires. The flow reactors provide superiorresults relative to batch processes due, at least in part, to areduction in homogenous nucleation and growth of nanoparticles in thefluid phase. The reduction in nanoparticle growth and the increase innanowire straightness may be optimized using the appropriate pressure,temperature, and flow rates. Without intending to be bound to anyparticular theory, the inventors believe the advantages of the flowreactor are that is provides (1) fluid-phase reactant concentrationsthat vary less significantly with time than other reactors due to theability to control reactant concentration flow rate; and (2) a means toflush particulates formed in the fluid phase before depositing on thesubstrate. Thus, continuous and semi-batch processes are capable ofproducing nanowires having very different and higher quality structuresthan their batch process counterparts.

FIG. 1 shows a schematic of a preferred embodiment for producing thenanowires. In FIG. 1A, a substrate is provided which comprises siliconhaving a silicon dioxide surface. The surface can be modified with asurface treatment to promote adsorption of a nanocrystal. Onto thismodified surface, the nanocrystal can be adsorbed. The nanocrystals canbe surface treated to provide for steric stabilization. In other words,as shown in FIG. 1A, tethered, sterically stabilized gold nanocrystalscan be used as seeds for further synthesis of nanowires, wherein thegold nanoparticles are adsorbed to the modified silicon substrate. FIG.1B provides an enlarged view of the selected region in (A), showingdimensions such as the length of the sterically stabilizing group, thesize of the nanocrystal, and the distance between the substrate surfaceand the nanocrystal. In FIG. 1B, this distance is occupied largely bythe coupling agent. In FIG. 1C, the degradation of diphenyl silane (DPS)to form Si atoms is shown. These Si atoms can dissolve into thenanocrystals to form alloy droplets on the substrate surface. In FIG.1D, the Si nanowires crystallize from the Au nanocrystal seeds uponsaturation of the particles.

Additional discussion for the present invention, including backgroundand theoretical discussion, can be found in “Growth of Single CrystalSilicon Nanowires in Supercritical Solution from Tethered Gold Particleson a Silicon Substrate” by Lu et al., NanoLetters, 2003, Vol. 3, No. 1,93-99, particularly the first four paragraphs, the entire disclosure ofwhich is incorporated herein by reference.

U.S. Patent Publication 2002/0172820 to Majumdar et al. (published Nov.21, 2002) discloses nanowires.

In addition, the following references can be used as guide to practicingthe present invention: (1) Madou, Fundamentals of Microfabrication,2^(nd) Ed., CRC Press, 2002 (for example, the properties and growth ofsilicon, including crystalline silicon, are described at pages 125-204);(2) “Control of Thickness and Orientation of Solution-Grown SiliconNanowires”; Holmes et al., Science, Vol. 287, Feb. 25, 2000, pages1471-1473 (this reference discloses bulk quantities of defect-freesilicon nanowires with nearly uniform diameters ranging from 40-50angstroms grown to several micrometers with a supercritical fluidsolution-phase approach); (3) “A Laser Ablation Method for the Synthesisof Crystalline Semiconductor Nanowires”; Morales et al., Science, Vol.279, Jan. 9, 1998, pages 208-211; (4) “Nucleation and Growth ofGermanium Nanowires Seeded by Organic Monolayer-Coated GoldNanocrystals”; Hanrath et al.; J. Am. Chem. Soc., Vol. 124, No. 7, 2002,pages 1424-1429; (5) U.S. Patent publication, 2003/0003300 A1 publishedJan. 2, 2003 to Korgel and Johnston, in particular, describingsupercritical fluid processes and use of organic silicon precursors toform silicon nanoparticles; and (6) “Supercritical Fluid-Liquid-Solid(SFLS) Synthesis of Si and Ge Nanowires Seeded by Colloidal MetalNanocrystals,” Hanrath, T. et al., Advanced Materials, 2003, 15, No. 5,Mar. 4, pages 437-440.

A nanowire is an individual nanowire, desirably a single crystal solidstructure. The length, diameter and aspect ratio (ratio of length overdiameter) of nanowires may vary over a considerable range. For example,the nanowires can have an average diameter of about 5 nm to about 50 nm,and more particularly, about 10 nm to about 20 nm. The length of thenanowires may vary, from a few microns to a hundreds of microns. In onetypical embodiment, the nanowires may have an average length of about atleast one micron, and more particularly, about at least 5 microns.Generally nanowire may be characterized by an aspect ratio of 100 orlarger and a diameter generally less than about 50 nm. For the purposesof this disclosure, the term nanowire also includes othernanometer-sized generally cylindrical structures that may go bydifferent names, such as nanorods, nanofilaments, or nanowhiskers.

Nanowires are a plurality of these individual nanowires, a collection ofsingle solid structures which in theory can be physically separated fromeach other and characterized individually. Statistical methods,therefore, can be used to characterize nanowires such as averagediameter, average length, and distributions. In many cases, narrowdistributions such as monodisperse distributions are more desired thanbroader distributions. In general, the individual nanowires have thesame composition when prepared from a common production method. Uponproduction of the nanowires, isolation methods can be used, if desired,to separate fractions of subsets of nanowires. Individual nanowires canbe isolated and characterized, and remixed if desired.

The nanowires can be crystalline including single crystal nanowires.Single crystal characteristics can be determined by high resolutiontransmission electron microscopy (HRTEM) analysis., In some embodiments,over 75 percent of the nanowires produced in accordance with the methodsof this invention are crystalline. This includes embodiments where over80 percent of the nanowires are crystalline and further includesembodiments where over 90 percent of the nanowires are crystalline.

The nanowires can comprise, for example, Group IV metals includingsilicon, germanium, tin, lead, and combinations thereof. The nanowirescan comprise at least 90 atomic percent, or at least 98 atomic percent,of the Group IV metal. Nanowires comprising silicon may be particularlydesirable.

The nanowires are desirably substantially straight. As such, they may besubstantially free of tortuous morphology such as kinks, curls, orbends, and substantially free of defects in the structure such asdefects in a crystal lattice. For the purposes of this disclosure, thestraightness of a nanowire may be characterized by a straightnessparameter which is the ratio of the shortest end-to-end length measuredalong the surface of a nanowire divided by the shortest distance betweentwo endpoints located at opposite ends of that nanowire. Using thisstandard, a perfectly straight nanowire has a straightness parameterequal to 1. The production process described by this invention can bevaried to provide substantially straight, and even completely straightnanowires (i.e. nanowires having a straightness parameter of about 1 orclose to 1 such as, for example, 1.1). In some embodiments the nanowiresproduced in accordance with the methods of the present invention willhave an average straightness parameter of no more than 3. This includesembodiments where the nanowires produced in accordance with the methodsof the present invention have an average straightness parameter of nomore than about 2 and further includes embodiments where the nanowiresproduced in accordance with the methods of the present invention have anaverage straightness parameter of no more than about 1.5. Depending onthe intended application of the nanowires, one skilled in the art candetermine whether the collection of individual nanowires has a lowenough content of nanowires which are not as straight as desired.

The nanowires (of a given composition) can be manufactured such thatthey are substantially free or totally free of nanoparticles of the sameor similar composition, in particular nanoparticles which can bepotentially formed as a competitive reaction during the formation of thenanowires. In some synthetic approaches, the formation of the nanowirecompetes with the formation of nanoparticles, and conditions arepreferred which result in nanowire production rather than nanoparticleproduction. For example, the nanoparticles are preferably produced in anamount of less than about 10 wt. %, and more preferably, less than about5 wt. %, and more preferably, less than about 1 wt. % of the total massof nanowire and nanoparticle production. When forming silicon nanowires,for example, the amount of silicon nanoparticle in the collection ofsilicon nanowires can be less than 5 wt. %, and less than 1 wt. % (basedon combined weight of nanowires and nanoparticles). Because thenanowires can be tethered to the substrate during formation, any unboundnanoparticles may be flushed away in the flow reactor. Additionally, thenanowires can be treated by, for example, washing with solvent to removefree nanoparticles, particularly when the nanowires are bound to asubstrate and the nanoparticles are not bound. For a given application,one skilled in the art can experiment and determine whether the amountof nanoparticle formation is too high.

In saying that the nanowires are free or substantially free ofnanoparticles, the following embodiment is not excluded: the nanowirescan include nanocrystals at the end which are physically part of thenanowire. The nanowires can comprise nanocrystals, preferably metallicnanocrystals, at the ends of the nanowires, preferably at only one ofthe two ends. This terminal nanocrystal can result from the syntheticapproach, wherein nanocrystals are used to seed the production ofnanowires. In one embodiment, substantially all of the individualnanowires comprise the nanocrystal at the end. Examples of nanocrystalsinclude gold, silver, iron, titanium, nickel, and aluminum. The size ofthe nanocrystal can determine the diameter of the nanowire which growsfrom the nanocrystal. Preferred sizes for the nanocrystal seed particleare within the range 2-25 nm in diameter.

Once the growth of the nanowires has proceeded to the desired extent,the nanowires may be removed (untethered) from the substrate usingsonication in a suitable solvent or washing under suitable conditions.This process also allows for control of nanowire length. Specifically,the length of the nanowires may be controlled by the level ultra sonictreatment that the wires are subjected to after synthesis. By increasingthe extent of the sonication, the average nanowire length can besubstantially reduced to a few microns from an initial average length ofhundreds of microns.

Any suitable continuous reaction process can be used to produce thenanowires of the present invention provided that process providessufficient control of the flow of reactant materials over the substrateto produce substantially straight, substantially defect-free nanowires,desirably with the substantial absence of nanoparticles. Thus, suitableproduction processes will include both continuous and semi-batchprocesses. A variety of suitable flow reactors are known andcommercially available, including those designed to withstand thetemperature and pressure of supercritical fluid conditions.

The process for preparing the nanowires according to the invention maycomprise a combination of steps. In an initial step, a substrate may beprovided which comprises nanocrystals, such as metallic nanocrystals, ornanocrystal precursors covalently attached (tethered) to the substratesurface using known reactions. Examples of suitable substrates,including substrate surfaces, include silicon, and germanium, mica andhighly ordered pyrolytic graphite. The substrate can be treated ifdesired to have a surface layer or coating, and either monolithic ormulti-layered structures can be used. For example, the substrate may besilicon or germanium coated with a thermal oxide, gold, or aluminum. Thenanocrystals or nanocrystal precursors may be tethered to the surfaceusing known reactions. For example, coupling agents can be used tocovalently attach the nanocrystals to the surface. Suitable couplingagents include, but are not limited to, silane coupling agents whichhave a reactive moiety which binds to the substrate surface and afunctional group which allows for attachment of the nanocrystal. Thethickness of the layer of coupling agent may vary. In some embodiments,the thickness of the layer will be about 5 nm or as small as 1 nm. Thesurface coverage of the substrate by the tethered nanocrystals ornanocrystal precursors can be controlled by controlling the reactionconditions under which the nanocrystals or nanocrystal precursors arebound to the substrate surface. In some embodiments, the surfacecoverage is desirably low. For example, the surface coverage by thetethered nanocrystals or nanocrystal precursors may be less than 10%,less than 5%, or less than 3%. The nanocrystals preferably arerelatively uniformly distributed on the substrate surface. Thenanocrystals preferably are not clumped together.

Nanocrystals or nanocrystal precursors can be contacted with thesubstrate surface in the form of colloidal dispersions, and thesuspended nanocrystal can be transferred to the substrate by adsorption.The nanocrystals can be sterically stabilized with the use of, forexample, adsorbed alkyl groups including C₅-C₁₀₀ alkyl groups, and moreparticularly, C₆-C₂₀ alkyl groups. For example, alkane sulfur compoundsincluding alkane thiols such as dodecane thiol can be used to stabilizethe nanocrystals. The steric stabilization layer can extend, forexample, about 1 nm to about 5 nm from the particle.

In an alternative step, the nanocrystals or nanocrystal precursors maynot be attached to a substrate in any way. The nanocrystals may befreely dispersed in a solution comprising at least one organic solvent.The solution may also contain Group IV metal precursor, or Group IVmetal precursor may be later exposed to the nanocrystals or nanocrystalprecursors during a reaction step under appropriate reaction conditions.

In a subsequent step, the process comprises continuously reacting thenanocrystals or nanocrystal precursors with Group IV metal precursor ina supercritical fluid environment. The Group IV precursor may compriseat least one Group IV organometallic compound and the supercriticalfluid may comprises at least one organic solvent. The organic solventprovides a supercritical fluid by heating the solvent above its criticaltemperature at a pressure above its critical pressure. The criticaltemperature and critical pressure for a fluid is known as the criticalpoint. One skilled in the art can determine what pressures andtemperatures are needed to achieve the supercritical fluid state for aparticular solvent system, without undesired degradation. Above thecritical point, neither a liquid nor gas state exists. Instead a phaseknown as a supercritical fluid exists. For example, a gas enters thesupercritical state when the combination of pressure and temperature ofthe environment in which the gas is contained is above a critical state.The critical temperature and pressure of other components may be readilycalculated or experimentally determined. Supercritical fluids may havehigh solvating capabilities that are typically associated withcompositions in the liquid state. Supercritical fluids also have a lowviscosity that is characteristic of compositions in the gaseous state.Additionally, a supercritical fluid maintains a liquid's ability todissolve substances.

Generally, the solvent may be any solvent with an accessiblesupercritical state that is capable of dissolving the chosen precursormolecules. Examples of solvents that may be used include, but are notlimited to, hydrocarbons, alcohols, ketones, ethers and polar aproticsolvents (e.g., dimethyl formamide, dimethyl sulfoxide, etc).Hydrocarbon solvents include, but are not limited to aromatic andnon-aromatic hydrocarbons. Examples of aromatic hydrocarbon solventsinclude, but are not limited to benzene, toluene, and xylenes. Examplesof non-aromatic hydrocarbon solvents include cyclic hydrocarbons (e.g.,cyclohexane, methylcyclohexane, cyclopentane, methylcyclopentane, etc.)and aliphatic hydrocarbons (e.g., pentane, hexane, heptane, octane,iso-octane, etc.). Octanol and cyclohexane are particularly suitableorganic solvents. The critical temperature of octanol, for example, is385° C., and the critical pressure of octanol is 34.5 bar. When octanolis subjected to temperatures and pressures above 385° C. and 34.5 bar,the octanol exists in a supercritical state. The supercriticaltemperature and pressure for cyclohexane are 280° C. and 40.5 bar,respectively.

The organometallic Group IV precursor can be a Group IV metal compoundthat includes organic groups. As used herein, a “Group IV metal”includes the elements of silicon, germanium, tin, and lead. Generally,organometallic Group IV precursors are compounds that may be thermallydegraded to form nanowires that are composed primarily of the Group IVmetal. In some embodiments, the nanowire contains a mixture of Group IVelements, such as Si_(x)Ge_(1-x), Si_(x)Sn_(1-x) or Ge_(x)Sn_(1-x).Organometallic Group IV precursors include, but are not limited toorganosilicon, organogermanium, organotin, and organolead compounds.Examples of Group IV precursors include, but are not limited to,alkylsilanes, alkylgermanes, alkylstannanes, alkylplumbanes,chlorosilanes, chlorogermanes, chlorostannanes, chloroplumbanes,aromatic silanes, aromatic germanes, aromatic stannanes, and aromaticplumbanes. Particular examples of organometallic silicon precursorsinclude, but are not limited to, tetraethyl silane, diphenylsilane, andmonophenylsilane. Particular examples of organometallic germaniumprecursors include, but are not limited to, tetraethylgermane ordiphenylgermane. When a substrate is used for the formation ofnanowires, the substrate is treated with the organometallic Group IVprecursor, even though the organometallic Group IV precursor beginsreacting and decomposing when heated. It may not be clear whether theprecursor has already fully decomposed by the time it reaches thesubstrate. When the nanocrystals are freely dispersed in solution ratherthan tethered to a substrate, the organometallic Group IV precursor ispreferably in solution with the nanocrystals prior to being exposed tothe supercritical fluid under reaction conditions. However, theorganometallic Group IV precursor and nanocrystals may also be separateduntil reacted in a supercritical fluid environment under appropriatereaction conditions.

The nanocrystal precursor may be any organometallic compound that reactsin situ to form the metal nanocrystals that will subsequently direct thegrowth of the semiconductor nanowires. For example, instead of addingcolloidal metal nanocrystals, such as Al nanocrystals, an organometallicprecursor, such as trioctylaluminum, may be added that will decomposeinto Al nanocrystals under appropriate reaction conditions. Examples oforganometallic precursors include, but are not limited totrialkylaluminum compounds (trimethylaluminum, triethylaluminum,tributylaluminum, trioctylaluminum, etc.), metal carbonyl compounds(Fe(CO)₅, Fe₂(CO)₉, Co₂(CO)₈), and gold salts with reducing agents(HAuCl₄/NaBH₄).

The surface of the nanowires of the present invention may be modified insitu during a continuous or batch process of this invention by theaddition of surface modifying agents during or after the formation ofnanowires. A number of modifications may be made, and one of skill inthe art may readily choose agents that modify surface characteristicsdepending on the particular application of the present invention. Forexample, agents may be chosen to passivate the surface of the nanowiresof the present invention. Surface-passivating agents include, but arenot limited to 1-alkenes, such as 1-hexene, 1-octene, 1-dodecene,1-hexadecene, 1-octadecene; alcohols, such as 1-octanol, 1-dodecanol;and thiols such as 1-hexanethiol, 1-octanethiol, 1-dodecanthiol. Example4 below shows a Ge nanowire passivated with 1-hexene. The Ge nanowire ofExample 4 shows very little surface oxidation after passivation with1-hexene.

As noted above, both continuous reactions and semi-batch reactions canbe used to produce the nanowires of the present invention. Exemplarycontinuous reaction processes are described in detail in the examplessections below. An example of a flow reactor for use in a continuousprocess is provided in FIG. 2. The details of one flow reactor areprovided in the working examples below. The elements of the flow reactorinclude a high pressure reaction cell, equipped with an inlet region andan outlet region. The reaction cell can be designed for loading in thesubstrate, allowing reactants to enter into the cell by the inletregion, allowing exhaust to exit the cell by the outlet region, andthermal control. The reaction cell can be equipped at one end, theoutlet region, with a back pressure regulator. At the other end, theinlet region, the reaction cell can be equipped with a preheater zone. Aplurality of lines can be used to allow the solvent, the reactive GroupIV metal compound, or both into the preheater zone.

A typical semi-batch process may be carried out as follows: nanowiresformed by the reaction of precursor molecules with tethered nanocrystalsare deposited on a substrate, such as a silicon substrate, locatedinside a suitable reactor, such as a Ti reactor. After synthesis thedeposited nanowire product material is cleaned by rinsing the reactorcell with supercritical solvent, such as hexane. The nanowires may thenbe removed by vigorous flushing with the supercritical solvent. At thispoint the substrate is effectively clean and subsequent reactions arepossible by recharging the reactor with reagents and repeating thesynthetic process. Alternatively, the conditions may be altered to allowfor post-synthesis modification of the nanowires. In this embodiment, asemi-batch process is used wherein the nanowires may be ejected from thereactor and collected in a receiving vessel in which wet chemistry isused to modify the nanowire surface chemistry. This may be accomplishedwithout exposing the nanowires to oxidizing atmospheric conditions.Surface functionalization of the nanowires may be accomplished bytransferring the nanocrystals (either on a substrate or in solution)under inert atmosphere (argon or nitrogen gas) into a vessel containingthe desired reagent(s) and solvent. Examples of suitable derivatizationreagents include alcohols (such as butanol, octanol, dodecanol, etc.),alkenes (such as octene, dodecene, hexadecene, etc.), chlorosilanes(such as octyltrichlorosilane), and thiols (such as octanethiol,dodecanethiol, etc.). Solution-phase chemical synthesis (describedbelow) is then used to derivatize the nanowire surface. This approachmay be desirable because it can be adapted to the synthesis of largequantities of nanowires by simply recharging the reactor with reagentsafter expunging the formed nanowire material. An example of abatch-process for use in functionalizing Ge nanowires is presented inexample 2, below.

Reaction temperatures, pressures, and times for both continuous andsemi-batch reactions may vary depending on the nature of the reactantsand the desired quality of the nanowires. The reaction temperature andpressure should be sufficiently high to provide a supercritical fluidreaction but not so high as to prematurely decompose the precursors. Oneskilled in the art can determine which temperatures and pressures, for aparticular experimental setup, provide the correct combination ofreactivity and lack of decomposition to provide the substantiallystraight crystalline nanowires which are substantially free ofnanoparticles. Typical reaction pressures for the supercritical fluidsmentioned herein may be about 10 MPa to about 100 MPa, moreparticularly, about 15 MPa to about 50 MPa. Typical reaction temperaturefor the supercritical fluids mentioned herein may be about 350° C. toabout 800° C., and more particularly, about 500° C. to about 600° C. Thereaction time is not particularly limited but can be optimized for agiven application. After reaction is terminated, the substrate can bewashed or flushed with substantially pure solvent, without precursor, toremove nanoparticles. Alternatively, the nanoparticles may be removedfrom the surface through sonication in a suitable solvent.

The concentration of the organometallic Group IV precursor can bevaried. In some embodiments, the concentration of the organometallicGroup IV precursor will be 0.1 M to 0.9 M, and more particularly, about0.1 M to about 0.4 M.

Flow rate of organometallic Group IV precursor across the substratesurface area can be an important parameter. In general, lower flow ratesare preferred. For example, a cross sectional flow rate parameter can beestablished by dividing the flow rate (mL/min) by the square centimetersof substrate area (cm²). Based on the working examples, for example,good results were achieved with flow rate of 0.5 mL/min and substrateareas of 5×20 mm (1 cm²). Cross sectional flow rates below about 1.0mL/min-cm² can be used.

Although the theory of the present invention is not fully understood, itis believed that modeling and kinetic studies can be used to helpexplain and practice the invention, particularly for the siliconembodiment. The transition between straight and tortuous wires dependson temperature. High temperatures tend to favor straight nanowires.Sufficient Si atoms can arrive at the crystallization sites to drive thegrowth of a 1-D crystalline silicon nanowire. If the growth site isstarved with respect to silicon atoms, or if crystallization is slow,the likelihood of incorporating a contaminant into the growing crystalto produce a defect becomes significant. Both increased temperature anddiphenyl silane (DPS) concentration enhance the Si atom supply rate,which favor straight wires. However, very high DPS concentrationsproduce large amounts of homogeneously nucleated Si particles inaddition to the heterogeneously nucleated nanowires. Presumably, thereexists an important precursor concentration above which theheterogeneous nucleation pathway leading to nanowires is overwhelmed byhomogeneous nucleation to form Si nanoparticles. By manipulating boththe DPS concentration and the flow rate, the flow reactor can enable theproduction of straight wires with minimal Si particulate formation onthe substrate. At a DPS concentration of 0.25 M and 500° C., the wiresproduced at a low flow rate of 0.5 mL/min in FIG. 5A were straight andclean, whereas those produced in the batch reactor were shorter andsurrounded by large quantities of Si particles (FIG. 4B). A kineticanalysis of the flow system explains qualitatively the ability toproduce straight wires with minimal nanoparticles in a flow reactor.

In other embodiments (see, for example, Example 3 below), a substrate isnot used, and the nanowires may simply be collected from the reactioncell. Sonication may still be used to control the length of thenanowires. The nanowires can be in a suitable solvent and not attachedto a substrate during sonication. Using the same principle as previouslydescribed, increasing the level of sonication can result in nanowireswith a decreased average length. In embodiments such as Example 3, theconcentration of the Group IV organometallic precursor can range from,for example, about 0.001 M to about 0.9 M. The molar ratio of the amountof Group IV organometallic precursor to metal nanocrystals can rangefrom 10,000:1 to 100:1, with 1000:1 being optimal.

In other embodiments as illustrated in Example 4 below, theconcentration of the Group IV organometallic precursor can range fromabout 0.001 M to about 0.9 M. The molar ratio of the amount of Group IVorganometallic precursor to metal nanocrystals can range from about10,000:1 to about 100:1, with 1000:1 being optimal. The amount ofsurface passivating agent is preferably no more than about 40% (byvolume with respect to the solvent), with about 10% being optimal.

Working Examples

In Example 1 below, nanowires were produced in accordance with thepresent invention using a continuous flow reactor. As a comparativeexample, nanowires were also produced using a batch process. The resultsdemonstrate that the continuous process unexpectedly produces superiornanowires. In Example 2 below, surface functionalized nanowires wereproduced in accordance with the present invention using a semi-batchprocess. In Example 3, Ge nanowires were produced using a continuousflow reactor in accordance with the present invention. Example 3 issimilar to Example 1, but the metal nanocrystals that direct nanowiregrowth are dispersed freely in solution rather than tethered to asubstrate as in Example 1. In Example 4 below, Ge nanowires wereproduced with simultaneous in situ passivation in accordance with thepresent invention. This example is similar to Example 3 except a surfacepassivating agent was added to the reaction solution containing the Geprecursor and the metal nanocrystals. In Example 5 below, Ge nanowireswere formed in a process similar to Example 3 except the metalnanocrystals were formed in situ using organometallic precursors ratherthan adding already-made nanocrystals to the reaction solution.

Example 1 Comparison of Continuous and Batch Production

A. Experimental

Si Substrate Preparation. A Si wafer (<100>, with thermal oxide 10 nm,Wafer World, Inc.) was cut into 5×20 mm samples that were degreased withdistilled deionized water (D—H₂O) and acetone in an ultra-sonic bath.These small Si substrates were immersed in a HCl/methanol (w/w=1:1)solution and then 98% H₂SO₄, each for 30 min. After rinsing with D—H₂Oand drying with N₂, the substrates were immersed for 1 h in a diluteaqueous solution of 1:1:40 (v/v/v) 3-mercaptopropyltrimethoxysilane,MPTMS (Gelest, Inc.)/D—H₂O/isopropyl alcohol to functionalize thesurface. The MPTMS-treated Si substrate was transferred to a colloidaldispersion of alkanethiol-coated Au nanocrystals in chloroform. The Aunanocrystals were synthesize according to the procedures described inthe literature (see references 22 and 23). This procedure was:

-   -   Dissolved 0.154 g HAuCl₄ in 15 mL D—H₂O and 1.114 g (C₈H₁₇)4-NBr        in 10.2 mL CHCl₃. Combined the solutions obtained and stirred        for 1 h. Collected the organic phase and added 100 microliters        of dodecanethiol (C₁₂H₂₅SH) into it while stirring. Dissolved        0.197 g NaBH₄ in 12.5 mL D—H20 and added the solution to the        stirring organic phase, too. Stirred 8 h. Collected the organic        phase, which was rich in gold nanocrystals.        After incubating for 2 to 10 h at room temperature, the        substrate was rinsed with and stored in D—H₂O for later use.

Nanowire Synthesis. Diphenylsilane (DPS, Gelest) was stored in an inertatmosphere under N₂. Feedstock solutions of DPS in anhydrous cyclohexane(Aldrich Chemical Co.) were prepared in a N₂ glovebox withconcentrations ranging from 0.1 to 0.9 M.

Batch reactions were carried out by loading an MPTMS and Aunanocrystal-treated Si substrate and 0.5 mL DPS feedstock solution intoa 1 mL titanium grade-2 cell (0.5 cm i.d., 2.0 cm o.d., and 7.0 cm longwith a titanium grade-2 LM6 HIP gland and plug, High Pressure Equipment,Inc.) in a N₂ glovebox. A brass block (7×25×17 cm) designed to hold upto six reactors was used to heat the reactor. The block was thermostatedwith a thermocouple (Omega, Inc.) and a temperature controller andheated by four 300 W ¼ in. diameter by 4.5 in. long cartridge heaters(Omega). The block was heated to the desired reaction temperature priorto inserting the cell. The cell was inserted into the preheated blockand reached the synthesis temperature within a few minutes with acalculated pressure (see reference 24) of 29.0 mpA. The reactionproceeded another 15 min at this temperature. A special cell filled witha thermocouple verified that significant temperature gradients do notoccur during the reaction and that the cell interior rapidly reachessynthesis temperature. The reaction was quenched by rotating the brassblock upside down with a cable and sliding the cell by gravity into anice-water bath. The cell contents cooled to less than 50° C. in twominutes. The entire device was shielded heavily with polycarbonatebarricades.

The flow reactor was a 2 mL (0.5 cm i.d., 2.0 cm o.d., and 12.5 cm long)high-pressure titanium grade-2 cell with both ends connected to 1/16 in.o.d. and 0.03 in, i.d. stainless steel high-pressure tubing via titaniumgrade-2 LM-6 HIP reducers (High Pressure Equipment) (FIG. 2.)Cyclohexane and a modified Si substrate were loaded into the cell underan inert N₂ atmosphere in a glovebox. Two stainless steel cylinders (1.7cm id., 2.5 cm o.d., and 20 cm long) were equipped with stainless steelpistons and ethylene propylene O-rings. In the glovebox, one of thesecylinders was loaded with cyclohexane and the other with DPS/cyclohexanestock solution. The cylinders were removed from the glovebox andconnected to the preheater tubing. The reaction cell was then removedfrom the glovebox and connected via a three-way valve to the preheatertubing. The preheater tubing and the reaction cell were covered withheating tape and glass wool insulation and heated from 300° C. to 350°C. and 350° C. to 500° C., respectively, in 3 to 5 min. The temperaturewas measured by thermocouples under the heating tape and controlled towithin about 5° C. in the preheater and 1° C. in the reactor. Thecylinder containing pure cyclohexane was pressurized by pumping D-H₂Ointo the back of the piston using an HPLC (high pressure liquidchromatography) pump (Thermoquest) to inject oxygen-free cyclohexanethrough the preheater tubing and into the reaction cell until reachingthe desired pressure. The valves to the first cylinder containing onlysolvent were closed and the DPS feed solution valves were opened. TheHPLC pump controlled the DPS solution flow rates, which ranged from 0.5to 3 mL/min. An SS-4R3A back-pressure regulator (Swagelok) connectedafter the reaction cell and a digital pressure gauge (Stratford) betweenthe preheater tubing and the cell maintained the pressure at 24.1±1.4MPa. The reaction proceeded for 5 min before switching the valves backto the solvent cylinder. Solvent was flushed through the cell at 3mL/min to remove undesired reaction byproducts and particulates from thesystem.

Materials Characterization. A LEO 1530 high-resolution scanning electronmicroscope (HRSEM) was used with a 10 kV accelerating voltage to studythe morphology of the nanowires on the Si substrate. X-ray photoelectronspectroscopy (XPS) was performed using a Physical Electronics XPS 5700equipped with monochromatic Al X-ray source (Al Kα, 1.4866 keV). Highresolution transmission electron microscopy (HRTEM) and selected areaelectron diffraction (SAED) were performed using a JEOL 2010F TEMoperating at 200 kV. Images were obtained primarily with a GATAN digitalphotography system. To avoid structural damage to the nanowires thatoccurs with sonication or solvent redispersion, samples were preparedfor HRTEM by scratching the surface of the silicon substrate withcarbon-coated 200 mesh Cu grids (Electron Microscope Sciences).

B. Grafting Nanocrystals onto a Surface

FIG. 3A shows an HRSEM image of a Si substrate after grafting Aunanocrystals to the surface. The average diameter of the gold particleswas 12.95±3.69 nm. In a control experiment, FIG. 3B shows the HRSEMimage of the Si substrate with gold particles after being subjected tothe reaction condition (T=500° C., P=24.1 MPa, flowing cyclohexane). Thegold particles have an average diameter of 13.09±2.67 nm. There was nosignificant change in the Au nanocrystal size distribution, indicatingminimal particle aggregation. This observation was consistent with thesiloxane monolayer being stable at least up to 815 K (see reference 25).FIG. 3C shows an XPS of a Si substrate after grafting Au nanocrystals tothe surface.

C. Batch Reactor Results

Although Si nanowires could be grown from the substrate under batchconditions, the nanowire quality was generally poor. FIG. 4 shows HRSEMimages of the Si nanowires grown in a batch reactor from Si substrateswith 2% Au nanocrystal surface coverage. At 500° C. and [DPS]=0.9 M,straight Si wires several micrometers in length were used observed (FIG.4A). However, the wires were heavily surrounded by Si particles (seereference 26). Lowering the DPS concentration reduced Si particleformation considerably ([DPS]=0.25 M; FIG. 4B); however, under theseconditions, most of the wires were shorter than 1 micron. Furtherreduction in DPS concentration ([DPS]=0.1 M) decreased particulateformation even more, but under these growth conditions, the wiresexhibit a tortuous morphology due to defects in the crystal lattice(FIGS. 4C and 4F). Reduced temperature continued to decrease Si particleformation (FIGS. 4D and 4E, [DPS]=0.25 M at 450 C and 400 C,respectively), but at the expense of exceedingly tortuous nanowireproduction.

D. Flow Reactor Results

A flow reactor provided the way to produce straight nanowires withminimal Si particulate formation, because it provides (1) fluid-phasereactant concentrations that vary less significantly with time and (2)ways to flush the Si particulates formed in the fluid phase beforedepositing on the substrate. Si Nanowires produced at 500° C.,[DPS]=0.25 M, and a feed rate of 0.5 mL/min were straight andcontaminated with very few Si particles (FIG. 5A). The inset in FIG. 5Ashows a gold particle at the end of a nanowire, indicating that the Auseeds participate directly in wire growth. The average Si wire diametermeasured by HRSEM, 16±4 nm, resembles closely the Au particle size andsize distribution prior to wire growth.

Au particles imaged by TEM appeared to be significantly smaller thanwhen imaged by HRSEM due to the difference in resolution limits of thetwo instruments. By TEM, the diameter is 6.7±2.6 nm, whereas HRSEM showsthat the nanocrystal diameter is 13.0±4 nm. The nanowire diametersdetermined by TEM match more precisely the nanocrystal diameterdetermined by TEM, while the nanowire diameter determined by HRSEMmatches the nanocrystal diameters measured by HRSEM.

Nanowires as short as 5 nm and longer than 10 microns were present inthe sample. Si particle formation increases at higher reactant flowrates. At 1.0 mL/min, particle formation became significant (FIG. 5B)and, at substantially higher flow rates (3 mL/min), nearly all thesubstrate ended up covered by Si particles (FIG. 5C). As in the batchreactor, reduced temperature resulted in bent and/or curly wires. Forexample, FIG. 5D shows curly wires formed at 0.5 mL/min at 450 C.Comparing the observed Si nanowire morphology resulting from differentreaction conditions, it was found that high temperature and low flowrates produced straight wires with minimal particle formation.

HRTEM reveals the straight Si nanowires to be single crystals. FIG. 6shows a representative image of a 7.2 nm diameter wire with the atomicplanes spaced by 0.31 nm corresponding to the (111) d-spacing of diamondcubic silicon. The single-crystal electron diffraction pattern recordedperpendicularly to the long axis of the nanowires (FIG. 6 inset) and thelattice resolved TEM images of the crystalline Si corresponds to a <110>nanowire growth direction.

Example 2 Surface Derivatization of Ge Nanowires

A Cl-terminated Ge nanowire sample was prepared. For background, see,for example, Lu, Z. H., “Air-stable Cl-terminated Ge(111)” Appl. Phys.Lett., 1996. 68(4): p. 520. Briefly, the oxide on the Ge nanowiressurface was removed with an HF etch (2%, 4 min) followed by a treatmentin diluted HCl (10%, 10 min). The nanowire sample was dispersed inethanol and transferred to a new Si substrate for subsequent XPSanalysis. A sulfide surface passivation was carried out. See, forexample, Lyman et al., “Structure of a passivated Ge surface preparedfrom aqueous solution,” Surface Science, 2000. 462: p. L594., wherein aHF etched nanowire sample was treated in an aqueous (NH₄)₂S solution at80° C. for 20 minutes followed by several rinses in ethanol. Nanowiresurface hydrogermylation was performed. For background, see Choi, K. andJ. M. Buriak, “Hydrogermylation of alkenes and alkynes on hydrideterminated Ge(100) surfaces,” Langmuir, 2000. 16: p. 7737. FIG. 7illustrates a representation of the surface chemistry occurring in thismethod. Briefly, the produced Ge nanowires were ejected from the reactorcell into a heated solution of 1-octene under an inert Ar environment.The solution was then refluxed at 70-80° C. for 2 h. Hydrogermylationexperiments have also been carried out in-situ by injecting 1-hexadeceneinto the Ti reactor cell containing the produced nanowires. The reactorcell was maintained at 210° C. for 2 h after which excess alkene wasflushed out of the reactor and the treated nanowires were transferred toan inert N₂ environment for storage. The derivatized Ge nanowires do notshow any signs of oxidation in energy dispersive X-ray spectroscopy(EDS) or HRTEM (FIG. 8). Derivatization of Ge nanowires was confirmed byFTIR spectroscopy (FIG. 9). The peaks at 2875 and 2910 wavenumbers aredue to C—H bond stretching and confirm the presence of alkyl groups onthe Ge nanowires. The absence of a peak at 3090 wavenumbers confirmsthat there are no C═C double bonds in the sample, providing evidence tosupport the hydrogermylation mechanism depicted in FIG. 7.

Example 3 Continuous Production—Metal Nanocrystals Not Tethered toSubstrate

Example 3 is similar to Example 1 described above. However, in Example1, the Au nanocrystals were tethered to a Si substrate. In this example,Au nanocrystals directing nanowire growth were dispersed freely insolution.

The flow reactor was a 2 mL (0.5 cm i.d., 2.0 cm o.d., and 12.5 cm long)high-pressure titanium grade-2 cell with both ends connected to 1/16 in.o.d. and 0.03 in, i.d. stainless steel high-pressure tubing via titaniumgrade-2 LM-6 HIP reducers (High Pressure Equipment) (FIG. 2.) Twostainless steel cylinders (1.7 cm id., 2.5 cm o.d., and 20 cm long) wereequipped with stainless steel pistons and ethylene propylene O-rings. Inthe glovebox, one of these cylinders was loaded with hexane and theother with 0.01 M diphenylgermane (DPG) and 0.00001 M Au nanocrystals inhexane stock solution. The cylinders were removed from the glovebox andconnected to the preheater tubing. The reaction cell was then removedfrom the glovebox and connected via a three-way valve to the preheatertubing. The preheater tubing and the reaction cell were covered withheating tape and glass wool insulation and heated to 300° C. and 500°C., respectively. The temperature was measured by thermocouples underthe heating tape and controlled to within about 5° C. in the preheaterand 1° C. in the reactor. The cylinder containing pure hexane waspressurized by pumping D—H₂O into the back of the piston using an HPLC(high pressure liquid chromatography) pump (Thermoquest) to injectoxygen-free hexane through the preheater tubing and into the reactioncell until reaching the desired pressure. The valves to the firstcylinder containing only solvent were closed and the DPG/Au nanocrystalreaction solution valves were opened. The HPLC pump controlled theDPG/Au nanocrystal solution flow rates, which ranged from 0.5 to 3mL/min. An SS-4R3A back-pressure regulator (Swagelok) connected afterthe reaction cell and a digital pressure gauge (Stratford) between thepreheater tubing and the cell maintained the pressure at 24.1±1.4 MPa.The reaction proceeded for 30 min before switching the valves back tothe solvent cylinder.

Example 4 Continuous Production with Simultaneous in situ SurfacePassivation

Example 4 is similar to Example 3 as described above. However, in thisexample, the nanowires undergo in situ surface passivation by adding asurface passivating agent to the solution containing the Ge precursorand the metal nanocrystals. In this example, 1-hexene was chosen as thesurface-passivating agent.

A. Experimental

The flow reactor was a 2 mL (0.5 cm i.d., 2.0 cm o.d., and 12.5 cm long)high-pressure titanium grade-2 cell with both ends connected to 1/16 in.o.d. and 0.03 in, i.d. stainless steel high-pressure tubing via titaniumgrade-2 LM-6 HIP reducers (High Pressure Equipment) (FIG. 2.) Twostainless steel cylinders (1.7 cm id., 2.5 cm o.d., and 20 cm long) wereequipped with stainless steel pistons and ethylene propylene O-rings. Inthe glovebox, one of these cylinders was loaded with hexane and theother with 0.01 M diphenylgermane (DPG) and 0.00001 M Au nanocrystals ina solvent consisting of 90% hexane and 10% 1-hexene. The cylinders wereremoved from the glovebox and connected to the preheater tubing. Thereaction cell was then removed from the glovebox and connected via athree-way valve to the preheater tubing. The preheater tubing and thereaction cell were covered with heating tape and glass wool insulationand heated to 300° C. and 400° C., respectively. The temperature wasmeasured by thermocouples under the heating tape and controlled towithin about 5° C. in the preheater and 1° C. in the reactor. Thecylinder containing pure hexane was pressurized by pumping D—H2O intothe back of the piston using an HPLC (high pressure liquidchromatography) pump (Thermoquest) to inject oxygen-free hexane throughthe preheater tubing and into the reaction cell until reaching thedesired pressure. The valves to the first cylinder containing onlysolvent were closed and the DPG/Au nanocrystal reaction solution valveswere opened. The HPLC pump controlled the DPG/Au nanocrystal solutionflow rates, which ranged from 0.5 to 3 mL/min. An SS-4R3A back-pressureregulator (Swagelok) connected after the reaction cell and a digitalpressure gauge (Stratford) between the preheater tubing and the cellmaintained the pressure at 24.1±1.4 MPa. The reaction proceeded for 30min before switching the valves back to the solvent cylinder.

B. Results

FIG. 12A-B shows HRTEM images of Ge nanowires produced in this exampleusing continuous flow synthesis with simultaneous in situ surfacepassivation.

In FIG. 10, FT-IR spectra indicated successful surface passivation withhexyl groups. The bottom line of FIG. 10 shows the unpurified Genanowires according the present example, and the top line shows the samenanowires after washing multiple times with chloroform and ethanol. Thepeak at 2965 wavenumbers is due to C—H bonds, indicating successfulsurface passivation. The same peak present both before and after washingindicates the surface passivation is stable and not removed by routinehandling or washing.

In FIG. 11, the Ge nanowires produced according to the present exampleshowed some surface oxidation. FIG. 11 shows FT-IR spectrum from thesame Ge nanowires both before (bottom line) and after (top line)washing. Both lines of FIG. 12 show a peak at 860 wavenumbers due toGe—O bonds indicating some surface oxidation, possibly due to incompletesurface passivation. The peak at 860 wavenumbers is unchanged afterwashing the Ge nanowires multiple times with chloroform and ethanolindicating that routine handling and washing does not change the extentof oxidation.

In FIG. 13, electron energy loss spectrum results (EELS) confirmed theresults obtained using FT-IR. FIG. 13 shows a large carbon signal (280eV) indicating surface passivation with alkyl groups, a small oxygensignal (530 eV) indicating some surface oxidation, and a germaniumsignal.

Example 5 Continuous Production with in situ Metal Nanocrystal Formation

Example 5 is similar to Example 3 described above. In Example 3, thereagent solution contained already-made metal nanocrystals to directnanowire growth. In Example 5, organometallic precursors,trioctylaluminum, reacted in situ to form the metal nanocrystals thatsubsequently directed the growth of the semiconductor nanowires.

The flow reactor was a 2 mL (0.5 cm i.d., 2.0 cm o.d., and 12.5 cm long)high-pressure titanium grade-2 cell with both ends connected to 1/16 in.o.d. and 0.03 in, i.d. stainless steel high-pressure tubing via titaniumgrade-2 LM-6 HIP reducers (High Pressure Equipment) (FIG. 2.) Twostainless steel cylinders (1.7 cm id., 2.5 cm o.d., and 20 cm long) wereequipped with stainless steel pistons and ethylene propylene O-rings. Inthe glovebox, one of these cylinders was loaded with hexane and theother with 0.01 M diphenylgermane (DPG) and 0.00001 M trioctylaluminumin hexane stock solution. The cylinders were removed from the gloveboxand connected to the preheater tubing. The reaction cell was thenremoved from the glovebox and connected via a three-way valve to thepreheater tubing. The preheater tubing and the reaction cell werecovered with heating tape and glass wool insulation and heated to 300°C. and 500° C., respectively. The temperature was measured bythermocouples under the heating tape and controlled to within about 5°C. in the preheater and 1° C. in the reactor. The cylinder containingpure hexane was pressurized by pumping D—H2O into the back of the pistonusing an HPLC (high pressure liquid chromatography) pump (Thermoquest)to inject oxygen-free hexane through the preheater tubing and into thereaction cell until reaching the desired pressure. The valves to thefirst cylinder containing only solvent were closed and theDPG/trioctylaluminum reaction solution valves were opened. The HPLC pumpcontrolled the DPG/trioctylaluminum solution flow rates, which rangedfrom 0.5 to 3 mL/min. An SS4R3A back-pressure regulator (Swagelok)connected after the reaction cell and a digital pressure gauge(Stratford) between the preheater tubing and the cell maintained thepressure at 24.1±1.4 MPa. The reaction proceeded for 30 min beforeswitching the valves back to the solvent cylinder.

The following references can be used as a guide to facilitate practiceof the present invention. By citing these references, the inventors donot admit that any of these references are prior art.

REFERENCES

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1. Crystalline nanowires comprising a group IV metal, which aresubstantially straight and substantially free of group IV metalnanoparticles; wherein each of the nanowires further comprises anon-group IV metallic nanocrystal at one end of the nanowire.
 2. Thenanowires of claim 1, wherein the group IV metal is silicon.
 3. Thenanowires of claim 1, wherein the group IV metal is germanium.
 4. Thenanowires of claim 1, wherein the nanowires have an average straightnessparameter of no more than
 2. 5. The nanowires of claim 1, wherein themetallic nanocrystals are gold nanocrystals.
 6. The nanowires of claim1, wherein the metallic nanocrystals are aluminum nanocrystals.
 7. Thenanowires of claim 1, wherein the nanowires have an average diameter ofabout 5 nm to about 50 nm.
 8. The nanowires of claim 1, wherein thenanowires have an average diameter of about 10 nm to about 20 nm.
 9. Thenanowires of claim 1, wherein the nanowires have an average length of atleast one micron.
 10. The nanowires of claim 1, wherein the nanowireshave an average length of at least 5 microns.
 11. The nanowires of claim1, wherein the nanowires are single crystals based on HR TEM analysis.12. The nanowires of claim 1, wherein the nanowires are single crystalsof silicon.
 13. The nanowires of claim 1, wherein the nanowires aresingle crystals of silicon, comprise metallic nanocrystals at their end,have an average diameter of about 5 nm to about 50 nm, and have anaverage length of at least one micron.
 14. A device comprising thenanowires of claim
 1. 15. A device comprising the nanowires of claim 14,wherein the device is a transistor.
 16. A device comprising thenanowires of claim 14, wherein the device is a field effect transistor.17. A device comprising the nanowires of claim 14, wherein the device isa sensor.
 18. A device comprising the nanowires of claim 14, wherein thedevice is a laser.
 19. A device comprising the nanowires of claim 14,wherein the device is a light emitting diode.
 20. A device comprisingthe nanowires of claim 14, wherein the device is a non-volatile memorydevice.
 21. The nanowires of claim 1, wherein the nanowires contain lessthan about 5 wt. % nanoparticles, based on the total mass of thenanowires and nanoparticles.
 22. The nanowires of claim 1, wherein thenanowires contain less than about 1 wt. % nanoparticles, based on thetotal mass of the nanowires and nanoparticles.
 23. The nanowires ofclaim 1, wherein the nanowires are disposed on a substrate comprisingcatalyst sites attached to the substrate surface.
 24. The nanowires ofclaim 23, wherein the nanowires are attached to the catalyst sites. 25.The nanowires of claim 24, wherein the attachment is a covalentattachment.
 26. The nanowires of claim 23, wherein the catalyst sitescomprise nanoparticulates.
 27. The nanowires of claim 23, wherein thecatalyst sites comprise metallic nanocrystals.
 28. The nanowires ofclaim 23, wherein the catalyst sites cover less than about 10% of thesurface of the substrate.